Roller-ramp and sprag clutches are one-way clutches that operate automatically and are capable of supporting high torque loads. However, metal fatigue limits the cycle life of roller-ramp and sprag clutches in high torque uses. The ramp angle in roller-ramp clutches and the strut angle in sprags must be shallow to achieve locking action. These shallow angles produce very high compressive force on the sprags or rollers, also called “grippers.” Because the force on these grippers is focused along narrow line contacts, high torque loading causes extreme pressure at the contacts. As a result, even the hardest steel alloys suffer metal fatigue after some number of high torque lock and release cycles. Cycle lifetimes of current technologies range from a few hundred thousand to a few million high torque cycles.
An advantage of roller-ramp and sprag clutch technologies is that the gripper elements are “self-locking.” That is, once friction contact is established the grippers lock automatically without need of any external forces or mechanisms. Biasing springs are commonly present in these clutches but only function to initiate friction contact between the grippers and the surfaces of the clutch races. When used in an application in which inertial forces reliably initiate friction contact no springs are required for the function of self-locking grippers. The Langen Overrunning Clutch (U.S. Pat. No. 67,659 August 1867), shown in FIG. 29, is an example of a roller-ramp clutch using no biasing springs.
Numerous attempts have been made to replace the rollers and sprags of conventional one-way clutches with wedge shaped gripper elements so as to increase the contact area and thereby decrease pressure and stress, thus minimizing metal fatigue. For example U.S. Pat. No. 8,020,681 includes an embodiment with large contact surfaces on wedge shaped grippers. But wedge grippers are not generally self-locking and require springs to press the wedges against the ramp surfaces to hold the torque load. The necessary spring force for maintaining high torque grip produces exceedingly high freewheeling friction. In addition, if the torque load momentarily exceeds the spring force the wedges lose grip, producing a dangerous runaway clutch failure. Runaway clutch failures do not occur in a one-way clutch with self-locking grippers.
U.S. Pat. No. 3,202,250 by Bertram Fulton discloses a wedge clutch with self-locking grippers. The patent reveals that application of a low-friction coating at the ramp surface can make a wedge-shaped gripper self-locking if the ramp angle is sufficiently small. FIG. 28 shows an embodiment of Fulton's disclosure in which the ramp surfaces are placed on the outer race for the expressed purpose of reducing freewheeling friction. With radially inward facing ramps centrifugal force urges the wedges away from the inner race during freewheeling rotation, reducing friction or even lifting the wedge off the counter-rotating race. The following friction analysis of Fulton's model is necessary for disclosure of the present invention's innovations.
FIG. 22 is a diagram of the forces acting on wedge 41 in a clutch of the same configuration as the model in FIG. 28. Fo is the sum of the pressure on the wedge from contacting ramp surface 45 of outer race 43. Fi is the sum of the pressure on the wedge from contacting circular inner race 42. During the locked state Fi and Fo must be equal in magnitude, opposite in direction and co-linear, and therefore angle θo is equal to θi. To produce self-locking action the wedge must slip at the ramp surface when the wedge slips against the inner race. For the wedge to slip at the ramp surface the ratio Fto/Fno must be greater than the coefficient of friction μo at the ramp, where Fto is the frictional component and Fno is the normal component of force Fo. Therefore angle φo must be greater than the friction angle arctan(μo) for locking action to occur. During slip at the inner race the ratio Fti/Fni is the kinetic coefficient of friction μi, where Fti is the frictional component and Fni is the normal component of Fi. Therefore the sum of angles (θi+φi) is equal to friction angle arctan(μi). It can be shown that combining these conditions leads to the following requirement for self-locking action:arctan(μo)<arctan(μi)−α−φi  (1)where α is the slope angle of the ramp. The larger the magnitude of φi the more difficult it is to comply with condition (1). But φi increases as the ratio Ro/Ri increases, where Ro is the rotational radius of Fo and Ri is the rotational radius of Fi. That is, the magnitude of φi increases with the thickness of the wedge. In practice φi often reaches a value that makes satisfying requirement (1) unattainable, or attainable for only small values of α. Therefore wedge-clutch designs with outer race ramps use thin spiral wedges and very shallow ramp angles. However a shallow ramp angle is known to lead to “lock up,” a state in which a wedge permanently jams between the races. Substantial force may be necessary to free up a locked up wedge. Additionally, wedge-clutch designs typically specify a circular ramp curvature or leave the ramp curvature unspecified. But a circular or undefined ramp curve does not distribute pressure evenly along the contacting surfaces, especially in spiral-type ramps. Instead these curvatures focus most of the compressive force on a small section of the wedge, leading to lock ups and fatigue processes.
Placement of the ramps on the inside race in a wedge-clutch design reverses the effect of the φi term in equation (1) and makes self-locking action more easily attainable. Most all wedge-clutch designs therefore use inner race ramps. But inner race ramps put the wedges in constant friction contact with the outer race during freewheeling rotation. This effect generates undesirable wear and friction, especially during long periods of high speed counter-rotation. High speed freewheeling is also known to cause freewheeling lockup in operation of inner ramp wedge-clutch designs. U.S. Pat. No. 9,016,451 discloses an inner ramp wedge-clutch with reduced freewheeling friction by use of a spring mechanism built into a wedge ring. Though reduced the design still requires significant freewheeling friction to initiate locking action and is still subject to freewheeling lockup. Other designs, for example U.S. Pat. No. 9,353,802, use an external actuator to disengage the wedges from the outer race during freewheeling operation. These designs do not operate automatically, however, and require additional and complex external mechanisms for operation of the clutch.